The present invention relates to a sensing or measuring apparatus which includes an optical sensor and to a method for using such an apparatus. More particularly, the invention relates to such an apparatus and method which involve an optical indicator in the sensor that is depleted or deteriorates with time or use.
Optical sensors are quite useful in systems to measure the concentration of a component in a medium. For example, optical sensors can be effectively employed for measuring or monitoring a given component in blood. Typical components measured by optical sensors include gases, such as oxygen and carbon dioxide, hydrogen ions (pH), electrolytes, glucose and the like. Certain of such optical sensors are disclosed in commonly assigned U.S. patent application Ser. Nos. 917,912 and 917,913, each of which is incorporated in its entirety by reference herein.
Such optical sensors include an optical indicator, e.g., a fluorescent dye, which interacts with the component to be sensed or measured. Typically, the optical indicator, often in combination with a component permeable matrix, is placed on or directly adjacent to the optical surface of an optical signal means, e.g., an optical fiber. The interaction between the optical indicator and the component to be sensed or measured is monitored utilizing optical signals carried by the optical signal means.
The accuracy of the component concentration measurement or determination made by systems including such optical sensors is dependent, among other factors, upon maintaining a relatively constant amount of active optical indicator in signal communication with the optical signal means. However, during fabrication and use of the optical sensor, at least a portion of the active optical indicator is often destroyed or lost, thus adversely affecting the integrity of the signals transmitted by the optical signal means as being indicative of the true concentration of the component in a medium. This problem is particularly troublesome in situations where it is desired to monitor the concentration of a component over a relatively long period of time.